This invention relates to spark timing controls for spark ignited, internal combustion engines and particularly to the type of control which generates normal spark timing pulses in response to one or more engine operating variables and further includes electronic apparatus capable of retarding the actual spark times from said normal pulses in response to another engine operating variable such as engine knock. An example of such a control is shown in U.S. Pat. No. 4,106,447 issued to Gene A. West on Aug. 15, 1978. The system described therein has demonstrated significant commercial success over the past several years in controlling spark timing to limit engine knock intensity in turbocharged and naturally aspirated, high compression engines mounted in mass produced vehicles.
This system is used with means for detecting the intensity of engine knock and generating a signal therefrom which specifies a desired degree of spark retard from the normal spark timing of the engine. This system includes apparatus effective to retard the spark pulses of the engine from the normal spark timing by a crank angle determined by the aforementioned signal. This system further includes minimum burn timer means to ensure, after each spark event, that the current remains cut off in the spark coil for a predetermined minimum time to ensure good combustion, even during high engine speed, maximum retard operation wherein the system might otherwise attempt to turn current back on in the spark coil before the greatly retarded previous spark event is finished.
The apparatus includes integrating means which integrates at a slower rate in one direction during most of the time between successive spark pulses but changes direction at the normal spark timing and integrates at a much faster rate back to a reference level, the crossing of the reference level triggering the actual spark pulse and once more changing the direction of integration. This apparatus provides a controlled crank angle of additional spark retard regardless of changing engine speeds, since the fast integration in the one direction is at a constant rate but the slower integration in the other direction is at a rate which varies with the amount of retard desired. Variations in engine speed affect the integration in each direction similarly so that the crank angle of additional retard is not undesirably affected by such variations.
However, most electronic circuitry is susceptible to stray radio frequency electromagnetic signals which have become increasingly common in recent years. It is therefore desirable to protect an electronic circuit to a reasonable degree against malfunction due to the possible receipt of such stray radio frequency (RF) signals.